The purpose of ocular and optical measurements is the mounting of lenses on frames that are worn. The mounting of lenses must provide an ideal correction as well as comfort of vision for the usual everyday activities (driving, reading etc.) which each represent different conditions of accommodation and position of the eye with respect to the lens. Several measurements are necessary for the fabrication and mounting of the lens.
Currently the quantities used are those of activities in far-vision, which correspond to a focus at infinity (>2 m), and in near vision (>30 cm and <2 m) and the following measurements:                measurement for single correction vision (or SV for Single Vision) for unifocal lenses:                    Pupillary Distance (PD): the pupillary distance is the distance between the centers of the pupils when the subject focuses at infinity. This measurement relates to the eye system only.                        measurements for strong and/or progressive correction vision (PV for Progressive Vision) for varifocal or progressive lenses: these measurements have the purpose of enabling the precise mounting of the lens by considering the frame that is worn. They therefore relate to the adjustment of the vision for a frame that is worn and adjusted for the face, by considering a focus at infinity perfectly aligned with the direction of the face.                    Monopupillary distance (MonoPD): distance between the projection of the pupil in the plane of the lens and the center of the frame. This quantity measures the horizontal off-centering to be applied during the edging of the lens.            Heights (Segment Heights): distance between the projection of the pupil in the plane of the lens and the bottom of the frame (presumed inside of the inner bezel). This quantity measures the vertical off-centering to be applied during the edging of the lens.                        
These measurements correspond to a sight of the eye at infinity. Near vision can be measured for distances considered to be fixed (40 cm), and they are generally deduced from tables.
In practice, the eye is placed in a situation of sight and tools make it possible to measure the place where the ray sighted by the eye intersects the lens. This makes it possible to implicitly take account of the sighting parameters of the subject: slight strabismus, capacity of convergence, of accommodation, and leading eye as a function of the position and orientation with respect to the point sighted in binocular vision.
As a function of the correction, the lenses are cut to manage sighting shifts due to the lens itself. For this, the lens-eye distance and the lens-eye angle in infinity focus configuration for a neutral head hold are taken into account. Here again nomograms are considered, with an average lens-eye distance of 13 mm and an average angle, called pantoscopic angle, of 10 degrees.
However, if these defects are significant, the measurement of near vision becomes necessary, while taking account of the sighting situation: reading, working on a computer etc., so many situations that have different sighting heights with respect to the face and that vary with the habits of the person.
Currently, the fabrication of high-tech lenses makes it possible to adapt vision at various angles and distances of sight. Each eye being different, and the measurement being defined to be carried out in the plane of the lens, it is therefore useful to have as many measurements as there are different sighting needs. In practice, the optician does not have time to take said measurements with conventional tools.
There are thus various sources of error which combine and add together during a mounting of lenses on a pair of spectacles:                The measurement error on the shapes of the frame: in general, the optician puts the pair in a machine that feels the inner bezel of the frame: error +−0.2 mm. For rimless frames, the size of the demonstration lens is measured or the optician chooses the shape with his or her client.        The lens edging error: the NF EN ISO 21987 standard gives maximum error values. Best practice production guides for opticians recommend in practice a maximum error of 1.5 mm for the error between the requested and the achieved horizontal centering, and of 1 mm maximum between the requested and the achieved vertical centering for progressive lenses.        The interpupillary measurement error: error committed on the PD, monoPD and height. No official recommendation, but practical values obtained in the order of the half-millimeter for the PD, and of the millimeter per eye for the monoPD and heights.        
There exist tables of maximum recommended errors on the final mounting as a function of the degree of correction and lens type. By subtraction, it can be deduced therefrom that the current interpupillary measurement qualities are adequate. But for this it is necessary that the measurements be performed correctly.